Abstract: The very good method to analyze the structural change of reaction process of polyvinyl alcohol modified UF resin by the traditional process alkali-acid-alkali is 13 C--NMR, and which extrapolate a possibly reaction. The structure analysis indicated that, under alkalinity condition, addition reaction was primary, reaction production was mainly monomethylolurea, the urea participated in the reaction completely, the formaldehyde was still in existence by polymerization, at the same time polycondensation reaction also happened. At a acid reaction stage, the polycondensation reaction was primary, the macro-molecule system was formed with the methylene or the ether linkage, the polymerization formaldehyde decomposed completely, and as the form of methylene glycol participated in condensation reaction. The third urea ioined might obviously reduce the free formaldehyde content of the reacting system, the reaction production was mainly monomethylolurea. The polyvinyl alcohol monomer chemical constitution had the good attribute on 13C--NMR, but the chemical shift that proved the polyvinyl formal didn’t examine, therefore in this experiment, the polyvinyl formal hasn’t been product.